The problem with ghostmare isn't just GotP+Nightmare. It's the other things it can pack.
Ghostmare has Steal and Reverse Time, two already annoying cards. So, when facing a Ghostmare, you face draw denial, unblockable damage, your permanents turned against you, your creature's effectively gone forever, and if you have no other permanents; your quanta producers removed. This makes games dreadfully boring and painful. Devtal may lock you down, but Ghostmare forces you to punch yourself in the face.
The second problem is Ghostmare packs the counter to it's counter (this is also the problem with Mono Aether). Ghostmare can steal sanctuaries that stop the nightmare effect, or hourglasses/sundials that give the player draw power, and then make use of either. So, you can't casually counter Ghostmare. The only way you can stop it is if you've got an exceptionally hard counter, the only ones I can think of being Monotime rush and Earth+Light EM Stall with PA's.
It's just unpleasent to go up against, and not creative.